As the data areal density in hard disk drive (HDD) writing increases, write heads and media bits are both required to be made in smaller sizes. However, as the write head size shrinks, its writability degrades. To improve writability, new technology is being developed that assists writing to a media bit. One approach that is currently being investigated is microwave assisted magnetic recording (MAMR), which is described by J-G. Zhu et al. in “Microwave Assisted Magnetic Recording”, IEEE Trans. Magn., vol. 44, pp. 125-131 (2008). Although MAMR has been in development for a number of years, it is not shown enough promise to be introduced into any products yet. In particular, a difficult challenge is to find a spin torque oscillator (STO) film that is thin enough to fit into the small write gap required for state of the art products while providing a high magnetic moment in the oscillation layer to generate a sufficient radio-frequency field on a magnetic medium bit for the assist effect.
STO devices are based on a spin-torque-transfer effect that arises from the spin dependent electron transport properties of ferromagnetic (FM1)-spacer-ferromagnetic (FM2) multilayers. When spin polarized current from the FM1 layer passes through the spacer and FM2 layer in a current perpendicular-to-plane direction, the spin angular moment of electrons incident on the FM2 layer interacts with magnetic moments of the FM2 layer near the interface between the FM2 layer and the non-magnetic spacer. Through this interaction, the electrons transfer a portion of their angular momentum to the FM2 layer. As a result, spin-polarized current can switch (flip) the FM2 magnetization direction if the current density is sufficiently high. STO devices may have FM layers with perpendicular magnetic anisotropy (PMA) where magnetization is aligned substantially perpendicular to the plane of the FM layer. However, unlike Magnetoresistive Random Access Memory (MRAM) where PMA is necessary to keep magnetization perpendicular to plane in a free layer and reference layer, for example, STO in MAMR and related applications has a sufficiently strong write gap field to align magnetization in magnetic layers without requiring inherent PMA in the FM1 and FM2 layers.
In a PMR writer, the main pole generates a large local magnetic field to change the magnetization direction of the medium in proximity to the writer. By switching the direction of the field using a switching current that drives the writer, one can write a plurality of media bits on a magnetic recording medium. Magnetic flux in the main pole proceeds through the ABS and into a medium bit layer and soft underlayer (SUL). In related HT18-031, we disclosed how a flux guiding layer (FGL) is FM2 in the aforementioned FM1/spacer/FM2 multilayer, and has a magnetization that is flipped to the opposite direction when current is applied between the MP and trailing shield (TS) and across a spin polarization (SP) layer (FM1) thereby generating spin torque on the FGL. As a result, there is increased reluctance in the write gap so that more magnetic flux from the MP will be concentrated in a direction orthogonal to the ABS to assist writing. Optionally, a FGL may also be formed in the side gaps and leading gap to prevent magnetic flux from leaking from the MP to the side shields and leading shield, respectively. However, an undesirable side effect is that the function of the side shields and leading shield in enhancing cross-track performance and down-track performance, respectively, is weakened. Thus, an improved PMR writer design is needed that enables a writing assist from one or more FGL to boost the write field without comprising tracks per inch (TPI) capability that is a measure of cross-track performance, or without degrading bits per inch (BPI) or down-track performance.